A powerful horse tranquiliser is being sold illegally in tablet form on the streets of a large British town, police have warned.

Lancashire police believe the blue pills being sold in Blackburn may contain ketamine.

On Friday, police said there had been two separate incidents where people reported having adverse effects from taking the tablets, which are believed to have been imported into Britain over the internet.

Police said the people buying these pills believe they are buying benzodiazepine, a prescription drug used to treat a variety of medical conditions.

However, they suspect the pills are not what they are being portrayed to be.

Detective Inspector Eric Halford, of Blackburn, said the information police had received about the tablets was very worrying.

“We know that they are being sold illegally as benzodiazepine, but our information indicates they may in fact contain ketamine.

“What is worrying about this is that if people who would normally take benzodiazepine take the same amount of these pills, they are potentially going to overdose and become very unwell.

“We have had anecdotal reports of people who are regular users of benzodiazepine taking just a fraction of the amount of these pills and becoming very ill.

“Fortunately, at this stage, the people who have been affected have received medical treatment and have recovered.

“I would always advocate people not to use any drugs other than those prescribed by a medical practitioner but, that said, we are fully aware people do, so I would like to urge them to take increased care and I would especially urge people not to buy drugs over the internet or take anything they are not 100 percent certain about the authenticity of.”

The pills being circulated are described as being blue on the outside but white inside. They are being sold in small snap bags with a picture of a cannabis leaf on it.

People who have taken these pills have displayed symptoms such as being difficult to rouse, and when roused were uncharacteristically aggressive.

Police are urging anyone who has information about the possession and circulation of these drugs to come forward with information by calling 101.